The schizophrenia spectrum disorders include schizophrenia (SZ), schizotypal personality disorder (SPD), and schizoaffective disorder (SD). Schizophrenia (SZ) is considered a clinical syndrome, and is probably a constellation of several pathologies. Substantial heterogeneity is seen between cases, which is thought to reflect multiple overlapping etiologic factors, including both genetic and environmental contributions. SD is characterized by the presence of affective (depressive or manic) symptoms and schizophrenic symptoms within the same, uninterrupted episode of illness. SPD is characterized by a pervasive pattern of social and interpersonal deficits marked by acute discomfort with, and reduced capacity for, close relationships as well as by cognitive or perceptual distortions and eccentricities of behavior, beginning by early adulthood and present in a variety of contexts.
Endophenotypes are quantitative, continuously distributed traits, symptoms or disease dimensions s typically assessed by laboratory-based methods or clinical observation. The use of endophenotypes allows complex psychiatric illnesses like SZ to be divided into more stable, readily definable categories that are more amendable to identification of clear genetic associations, as they are generally more reflective of specific underlying biological processes. Identifying the genetic basis of specific endophenotypes also facilitates identification and development of new drugs that target the specific physiological deficits underlying disease. See Braff et al., Schiz. Bull. 33(1):21-32 (2007).